In a yarn accumulation and feeding apparatus of the type to which the invention relates, a switching means serves for controlling the drive responsible for the replenishment of the yarn supply. As the magnitude of the yarn supply decreases to a minimum, the yarn supply is again increased, eventually up to a maximum magnitude, whereupon the drive is again deactivated. The control process may also be accomplished in such a manner that the replenishment of the yarn supply is carried out so as to keep the latter between its maximum and minimum values without actually attaining one of these values. If in this case the magnitude of the yarn supply attains either its maximum or its minimum value, this is taken as an indication of a disorder and results in the switching device deactivating the drive not only of the yarn accumulation and feeding apparatus, but also of associated machinery cooperating therewith. Yarn accumulation and feeding devices with a stationary accumulating body usually employ a finger-shaped probe element secured outside of the accumulating body and having one of its ends projecting into an axial recess of the accumulating body under the bias of a spring. The probe element either cooperates with a switching device or is itself provided with contacts for generating a signal in cooperation with complementary contacts. As the yarn supply grows to a certain magnitude, the probe element is deflected, resulting in the generation of a signal for interrupting the further increase of the yarn supply. This known apparatus suffers from the disadvantage that the probe element exerts a certain force on the yarn windings, resulting in a perceptible jerk on the withdrawal of each winding. In the yarn processing or textile industry it is essential, however, that the tension of the supplied yarn not only be very low, but also be substantially constant, which is the main object of the yarn accumulating and feeding device. As the yarn is being withdrawn, it should form a so-called balloon, which is prevented, however, by the exteriorly located probe element. It is further disadvantageous that the probe element, or its spring bias, has to be adjusted in conformity to the type and properties of the yarn to be fed as well as in conformity to the thickness of the yarn, which requires a rather complicated structure. These circumstances led to the increasing use of optical or optoelectronical sensing devices capable of scanning the yarn supply without actually contacting it. In one of these devices, a light beam is directed onto a stationary reflecting surface on the accumulating body so as to be reflected thereby, the reflected beam being then monitored as to its intensity or the like. It is very difficult, however, to consistently generate a strong and significant signal capable of being so monitored.
It is therefore an object of the present invention to provide a yarn accumulating and feeding apparatus of the type defined above in which the control or monitoring of the magnitude of the yarn supply is accomplished without interfering with the yarn withdrawal from the accumulating body.